1. Field of the Invention
The present invention relates to a tracing table capable of simultaneously tracing two independent paths on a same tracing plane, of the type comprising:
a tracing plane, PA1 two arms parallel to each other and to the tracing plane, these two arms being disposed on the same side of said tracing plane and movable in a direction X of the tracing plane while remaining parallel to a direction Y of said plane, PA1 a carriage movable along each arm, PA1 a tracing ink pen or scriber supported by each carriage, PA1 and a lifting mechanism associated with each ink pen and adapted to raise the relevant pen to a limited extent. PA1 When the exchange is made for paths having very different Y.sub.1 and Y.sub.2 ordinates, one must wait until each pen has accomplished a relatively long travel along the axis of the arm supporting this particular pen before it can resume the tracing operation, and this causes a relatively considerable portion of the record to be lost irremediably. PA1 The exchange should be started before the difference between the abscissae X.sub.2 -X.sub.1 has become too small, to prevent the pens from meeting each other when the ordinates Y are exchanged. As a result, a relatively great exchange frequency is observed. PA1 if the two paths have relatively close abscissae X.sub.1 and X.sub.2 during a certain time, exchanging the pens will prevent them from resuming the tracing operation due to the insufficiency in the new difference between the abscissae, and another exchange is started, thus causing the device to oscillate continuously. PA1 If the electric means provided for controlling the exchange between the abscissae movements fail to operate for any reason, as far as the arm driving motors are concerned, or if this control action takes place with a certain delay, a shock will take place between the arms, and costly mechanical damages may result, especially if the table has relatively large dimensions affording relatively high-speed movements of the pen-supporting arms. PA1 In case the two diagrams are to be traced with different colors, the possibility of substituting on each arm a pen of one color for a pen of another color should be preserved. Now this exchange frequently applies to two pens movable on two paths inclined to each other and crossing each other at the point of operation. As a result, the accuracy of the tracing will depend on the height of the arm above the tracing plane. On the other hand, this arrangement increases appreciably the hidden area about the diagram portion traced at a given time.
2. Description of the Prior Art
So-called 2X--2Y tracing tables are already known which are capable of tracing simultaneously two diagrams or charts of rectangular coordinates on a common support, the four variables deriving for example from a same phenomenon that can be observed.
On certains tables of this type, each diagram is traced by a pen carried by an arm. The dimension Y of a point of this diagram is defined by the position of the pen along the longitudinal axis of this arm, and the corresponding dimension X is defined by the position of the arm which is movable parallel to itself.
It is also possible to trace a diagram on each one of the two faces of the support, as disclosed in the French Pat. No. 2,237,538, but this arrangement involves certain servitudes in connection with the rigidity and transparency of the support.
If the two arms of the table are disposed on a same side and at a same level above the tracing plane, it is obvious that some contrivance has to be used when the two arms operate close to each other and are likely to interfere with each other.
The so-called "Auto-Reverse" or pen-exchange device is well known in the art. It comprises a mechanism capable of changing over the pen control means so that, after the change-over, the tracing of each diagram is assumed by the pen having previously made the other diagram. Of course, during the transfer operation the pen should be lifted by a pen-lifting mechanism which, in any case, must be provided, were it only for returning the pens to their initial or starting positions.
However, this pen exchange device is attended by the following shortcomings: